1. Field of the Invention
The present invention relates to methods and apparatus for producing vapor/gas mixtures, and more particularly to the production of mixtures containing different vapors and at various concentrations.
2. Description of the Prior Art
Gas phase analyzers are used for many applications including the analysis of gas emissions generated in various chemical processes, and are particularly useful for monitoring compliance with environmental regulations. These gas phase analyzers must be calibrated, requiring the generation of mixtures of the vapor(s) to be monitored in a dilution gas with precise concentrations.
By current methods, production of the requisite vapor/gas mixtures, and therefore the subsequent calibration of analytical instrumentation using such mixtures, is expensive, fraught with hazards, and not entirely satisfactory. For example, commercially available, custom mixtures of gases containing known quantities of a vapor or gas to be analyzed by a gas phase analyzer are contained in very heavy cylinders which must be secured and which are difficult to transport. Further, only a few of the volatile compounds used in industry are available as custom mixtures, either because of toxicity, danger of condensation at the desired concentration, or unfavorable economics. Also, the accuracy and/or stability of such mixtures has been problematic. Moreover, when many volatile compounds are to be monitored, a separate tank or container is required for each compound, a regulator is required for each tank, and a manifold is required to introduce the appropriate mixture into the analyzer. Vapor/gas mixtures suitable for calibrating gas phase analyzers available at low concentration, in the order of parts per million concentrations, are often generated from high concentration mixtures using gas blending equipment. The high concentration mixtures used in this method have the same requirements and suffer the same problems as mentioned above, and there is an additional expense in the purchase of the blending equipment. An alternative to storing the needed vapor/gas mixtures in cylinders is to generate the mixtures dynamically when they are needed. Diffusion tube and permeation tube generators are very accurate and many solvent mixtures of interest can be generated. The concentration range which can be generated is from parts per billion to high parts per million, depending on the characteristics of the volatile compound, the oven temperature, and the flow rate of the dilution gas. Unfortunately, diffusion and permeation tube calibrators are not automated and thus are labor intensive, do not accommodate more than a few volatile compounds, and cannot generate concentrations in the percent range which is frequently desired in certain process and environmental analyses.
Vapor/gas mixtures are also produced in the prior art by bubbling a gas through a liquid. If the system is well designed, and the temperature and pressure of the system are well controlled, a mixture of the vaporous compound, at its vapor pressure, and the gas is generated. The bubbler method can be automated fairly easily, but the temperature must be very accurately controlled and the concentrations produced can vary over a wide range for solvents of slightly different boiling points. Only one concentration can be produced, and for typical solvents the concentration is quite high.
Highly accurate concentrations of vapor/gas mixtures are also commonly prepared by introducing a measured mass of volatile compound into an inert enclosure, and then adding an inert gas to the vapor until the desired final total pressure is obtained. Since an accurate automated method for measuring and transferring a small mass liquid is not available, this method is not suitable for an automated vapor/gas generator. Commercial vapor/gas mixture vendors prepare mixtures in essentially this manner. A measured mass or volume of the solvent of interest is injected into a cylinder and then the cylinder is filled with permanent, dilution gas to a final measured pressure and temperature.
What is desired is a method to produce accurate, custom vapor/gas mixtures on demand, without the need to change, store, or transfer large gas cylinders.